Apparatus And Method For Solvent Recovery From Drying Process

ABSTRACT

Method and apparatus for condensing a majority of the solvent in a process gas stream at low temperatures, e.g., below the freezing point of water, ca. −5° C. The gas stream exiting the condenser step may be further processed in one or more emission control devices, such as a single or multi-step series of concentrator devices, such as zeolite concentrator devices. One or more emission control operations can be carried out downstream of the single or multi-step concentrators. The aforementioned condensing process enables the one or more concentrators to operate in a favorable temperature range for removal of 99% or more of VOC, thereby meeting or exceeding strict environmental regulations.

This application is a continuation-in-part of U.S. Ser. No. 16/508,482 filed Jul. 11, 2019, the disclosure of which is incorporated herein by reference.

BACKGROUND

In the manufacture of lithium ion batteries and the like, a wet coating may be applied to a substrate in a slurry or paste form and is composed of fine powders mixed with a binder material, typically a polymer which may be water-soluble. In some cases, the binder is dissolved in an inorganic solvent such as N-methyl-2-pyrrolidone (NMP), acetone, various alcohols or similar industrial solvent selected to dissolve the organic binder to form a coatable liquid. These web-based products may be coated on at least one face (side) of the substrate web. In these cases, a wet coating is applied continuously or discontinuously on a moving web and dried in an oven or dryer to remove solvent thereby solidifying the applied coating. Throughout this application a continuously applied coating shall be understood as applying a coat to a substrate, like a web, foil, or the like in continuous process with or without timely and/or locally varying coating parameters (e.g. thickness, chemical composition and/or physical parameters of the coating material, etc.), whereby this continuous process shall also include the coating in regular or irregular patterns on a substrate continuously moved through the coating process.

In certain cases, the aforementioned web-based products are coated on both faces (side) of the substrate web. In typical cases wherein both faces of the substrate web are to be coated, a first web coating is applied continuously on a moving web and dried in an oven or dryer followed by application of a second wet coating which is subsequently dried in a second drying step.

In a preferred embodiment for production of battery electrodes, the wet slurry is applied to both sides of the foil web and subsequently dried in an oven or dryer. This arrangement is referred to as simultaneous dual-side coating and drying. In the case of lithium ion electrode manufacture, this arrangement is particularly advantageous in enhancing productivity, needing only one drying step following application of the wet coating slurry to both side.

Recovery of VOC solvent emissions from these and other industrial process, such as solvents condensable at operating temperatures of typical chilling systems (e.g., N-methyl-2-pyrrolidone (NMP), triethyl phosphate (TEP), dimethylacetamide (DMAc)), or other condensable fluids from an industrial operation such as the manufacture of lithium ion battery electrode, or drying and curing of polymeric films typically involves condensing the volatile organic compounds (VOC) along with water and other potential contaminants in low temperature dehumidification or condenser coils having finned surfaces followed by mist removal devices. In most web drying operations, at least a portion of the air exiting the condensing step is vented to atmosphere in a side stream while the balance of the air may be returned to the dryer. The flow rate of said side stream is at minimum equal to the fresh air entering the drying equipment, notably the infiltration air of the web slots. In the case of many commonly used solvents such as NMP, the concentration exiting the condenser operation is well above accepted limits for discharge into the atmosphere. In addition, recovery of such solvents may be cost-effective and desirable. Hence additional downstream emission control operations (emission control devices) are generally necessary to reduce the VOC concentration to permissible levels. VOC capture and or destruction methods include thermal (catalytic and straight thermal) oxidizers, scrubbers, carbon adsorption and adsorption on concentrator medias such a zeolite. Acceptable emission concentrations for VOC discharge to atmosphere are typically in the range of 10 to 20 mg/Nm³ as carbon, although in many locations these concentrations are being reset to lower limits with greater frequency.

Current market conditions for the production of green energy goods such as electrodes for batteries demand a much lower concentration level of VOC emissions, on the order of 1 mg/Nm³ or less. Conventional emission control devices are impractical choices in reaching these low levels. Moreover, energy consumption is high for most of the conventional emission control methods and some or all of these devices as currently configured in the marketplace are incapable of reaching such low outlet emission concentrations. VOC capture operations with condensing coils require extremely low temperatures to reach equilibrium vapor pressures necessary to emit sufficiently low VOC concentrations exiting the condensing unit. For instance, with NMP in the incoming air stream, in order to reach 1 mg/Nm³ in the effluent stream the condensing coils would need to run with coil surface temperatures below −35° C. Most drying processes also include water vapor in the dryer exhaust along with the VOC species. Such temperature conditions often result in freezing of water on the coil fins and tubes building ice, which eventually blocks the airflow passages between tubes and the fins in the heat exchange core of condensing coil thereby requiring a thawing to melt and remove the ice blocking the coil. In order to operate continuously, some systems may be arranged with two or more condensing coil sets in parallel. Additional valves, heaters and air moving hardware are provided such that one or more condensing coil path is on line in condensing operation mode while at least one condensing coil path is isolated from the effluent flow path and is operating in thaw mode. Issues of reliability and energy efficiency often plague such systems to the point they are avoided altogether.

Accordingly, an apparatus and method of reducing or eliminating such VOCs that does not suffer from the drawbacks of the prior art would be highly beneficial.

SUMMARY

Problems of the prior art have been addressed by embodiments disclosed herein, which provide a method and apparatus for overcoming the limitations of the prior art in an innovative and useful way by condensing a majority of the solvent in a process stream at low temperatures (e.g., even below the freezing point of water, ca. −5° C.) compared to conventional thermal coil condensers. The remaining solvent in the gas stream may be below 50 mg/Nm³ owing to the low temperature condensing step. In this temperature range solvents such as NMP or DMAc exhibit anti-freeze behavior depressing the freezing point of the water-solvent mix thus avoiding buildup of ice. In certain embodiments, the gas stream exiting the condenser step is further processed in one or more emission control devices, such as a single or multi-step series of concentrator devices, such as zeolite concentrator devices. The aforementioned condensing process enables the one or more concentrators to operate in a favorable temperature range resulting in the removal of 90 to 99% or more of VOCs, thereby meeting or exceeding strict environmental regulations.

Concentrator flow capacity per given volume of adsorbent media is generally higher at lower removal percentages and is reduced at higher removal percentages. Therefore the optimal design point for percentage removal of VOC's for a particular solvent laden air stream in each stage being handled with two or more emission control devices in series, the first being a concentrator type, may be in the lower portion of the range of 90 to 99% removal. In one example embodiment, a first step concentrator removes 90% of VOC, leaving less than 10% of the incoming amount, e.g., leaving 5 mg/Nm³ when the incoming amount is 50 mg/Nm³. An optional second step concentrator device also removes ≥90% of the incoming VOC from said first step concentrator. The resulting exit concentration is therefore on the order of about 0.5 mg/Nm³, meeting strict environmental regulations, including the new goals of the battery industry, for example.

Accordingly, certain embodiments disclosed herein relate to a circulation air conditioner such as for a recirculating air dryer or dryers generating circulation air laden with at least one condensable fluid (such as NMP), the circulation air conditioner comprising:

at least one main condenser having a feed port and an exhaust port and at least one main condensation stage, comprising:

-   -   i. a condensation chamber being accessible by or permeable for         the circulation air, and     -   ii. a cooling coil at least partially arranged inside said         condensation chamber and permeated by a cooling medium,     -   iii. whereby the cooling coil of said main condenser is operated         with a main cooling medium temperature of 0° C. or less,     -   b. a circulation air feed line being connected to said feed port         of said main-condenser and being connectable to a source of         condensable fluid laden air such as an exhaust circulation air         duct of said dryer or dryers for carrying the circulation air,     -   c. a circulation air exhaust line being connected to said         exhaust port of said main condenser and being connectable to a         feed circulation air duct of said dryer or dryers, and     -   d. a side-stream off gas extraction line being fluidly connected         to at least said condensation chamber of said main condenser,     -   i. whereby a volume flow of a circulation air streaming in said         condensation chamber is split into a high volume re-circulation         stream leaving the condenser through the circulation air exhaust         line and a low volume off-gas side stream.

In certain examples, the side-stream and the re-circulation stream are split in volume by a split-ratio between 0.1% and 20%, preferably between 0.5% and 10%, and even more preferably between 1% and 5%.

In certain aspects, the circulation air conditioner may further comprise at least a pre-condenser with at least one pre-condensation stage that is placed in the circulation air stream upstream of the main condenser and comprises a pre-condenser condensation chamber being accessible by or permeable for the circulation air, and a cooling coil at least partially arranged inside said pre-condenser condensation chamber and permeated by a pre-cooling medium, whereby the pre-cooling medium has a temperature higher than the main cooling medium temperature.

The pre-condenser and said main-condenser may be enclosed in a common condenser housing.

The circulation air conditioner of any of the foregoing embodiments, alone or in combination, may further comprise a pre-cooling heat exchanger arranged upstream of the pre-condensation stage or at least upstream the main condensation stage to already reduce a temperature of the streaming in circulation air and/or a reheating heat exchanger being arranged downstream of the main condensation stage. The pre-cooling heat exchanger and the re-heating heat exchanger may be thermally coupled by the exchange of a heat transfer medium such as water, brine or suitable thermal fluid and/or thermally coupled by a thermocouple or heat pipe. In some examples, the pre-cooling heat exchanger and the re-heating heat exchanger are in addition or alternatively thermally coupled via at least one thermocouple or heat pipe.

The circulation air conditioner of any of the foregoing embodiments, alone or in combination, may further comprise an air pollution control unit fluidly connected to the side-stream off gas extraction line and having at least one adsorptive concentrator with a gas exhaust and a desorption exhaust as a first pollution control stage and at least a second pollution control stage being selected from the group may comprise an adsorptive concentrator that is fed by the gas exhaust of the first pollution control stage and having a gas exhaust and a desorption exhaust. The desorption exhaust of at least one of the adsorptive concentrators may be connected to a desorption consisting of a filtration device, an absorptive concentrator, a thermal oxidizer, and a catalytic device. The second pollution control stage line, which is connected to a desorbate condenser, whereby a gas exhaust of the desorbate condenser is fed back into the side-stream off gas extraction line. The second pollution control stage may comprise at least one activated carbon filter.

The circulation air conditioner of any of the foregoing embodiments, alone or in combination, may further comprise one or more suitable sensors and one or more injectors or the like to introduce humidity to the solvent-laden gas stream, such as in the circulation air feed line. The one or more sensors and the one or more injectors may be part of a control system, which may include a PID controller, to dynamically control the humidity of the solvent laden gas stream.

In its method aspects, embodiments disclosed herein relate to a method for conditioning a circulation air laden with at least one condensable fluid, comprising:

-   -   a. providing the circulation air at a first volume flow and         intake temperature level well above 0° C. to a main condenser         with at least one main condensation chamber;     -   b. gradually cooling the circulation air to a main temperature         level of 0° C. or less inside the main condensation chamber;     -   c. splitting the volume flow of the circulation into a high         volume re-circulating flow and low volume off-gas side stream         after reaching a second temperature level; and     -   d. providing the high-volume re-circulating flow to a         circulation air intake of a dryer.

In certain aspects, the method may further comprise providing a condenser to cool the circulation air according to step b), which comprises at least one cooling coil filled with a cooling medium, and whereby the cooling medium enters the coiling coil at the far end side of a circulation air entry with cooling medium entry temperature of 0° C. or less and is heated while traveling through the cooling coil preferably in a counter-flow direction to the circulating air. The flow and temperature of the entering cooling medium are preferably measured with a suitable flow meter device and temperature sensor such as a resistance temperature detector (RTD) respectively. The temperature of the exiting cooling medium is preferably measured with a suitable temperature sensor such as a resistance temperature detector (RTD). The circulating flow may be driven by a fluid pump in communication with the fluid entry connection of the cooling coil. Further, the temperature of the air exiting the cooling coil is preferably measured with an array of one or more temperature sensors spaced across the cross-section of the exit face of the coil. Said air temperature sensors may be, for example, RTD's or thermocouples. The coolant flow rate and temperature of the entering cooling media may be controlled to pre-determined respective set points by suitable PID controllers in control communication with a flow control device, preferably a variable speed centrifugal pump and a temperature control valve, preferably a three-way flow proportioning valve set positioned by an actuator. Coolant is admitted to the circulating flow conduit from the chilled brine source, typically a water-cooled or air-cooled centrifugal chiller. Said three-way flow path valve set allows fresh cooling media from the cooling media source to enter the circulating flow path through the coil while an equivalent flow of heated cooling media exiting the cooling coil return connection is discharged back to the cooling media source. In some embodiments, the temperature of the air exiting the cooling coil may be controlled to a pre-determined set point by a suitable PID controller in control communication with a valve and actuator positioned in the conduit admitting fresh coolant from the chilled brine source. The pre-determined set point for the air exiting the cooling coil is selected by the operator to reach the desired target concentration of solvent or aqueous humidity of the air exiting the coil as determined from theoretical and or empirical vapor pressure equations, such as the Antoine Equation and Raoult's Law or engineering simulation model tools such as ChemCad, ASPEN, or other theoretical or empirical methods known in the art, or preferably from experimental test measurements. Alternatively, in a preferred embodiment the output from the exit air temperature measurement is input to a first PID controller configured to calculate by control algorithm the temperature set point for the entering cooling media. Accordingly, the output of said first PID controller is in communication with a second PID controller which receives the temperature set point from said first PID controller in a cascade control arrangement between the two respective PID controllers. The second PID controller output positions the actuator of said three-way flow path valve to control the measured temperature of the cooling medium entering the coil to the input set point of said first PID controller. In this cascade arrangement, the output of the first controller and therefore the set point of the second controller is available in temperature units as an engineering parameter which can be bounded between allowable minimum and/or maximum set point values in accordance with the functional capability of the coolant system. This parameter can also be translated into other engineering parameters by calculation, including but not limited to expected solvent and water concentrations in the air film boundary of the coil fins and tube surfaces according to vapor-liquid equilibria (VLE) relations such as the Antoine Equation and Raoult's Law or other theoretical or empirical methods known in the art, or from experimental test measurements. These parameters are preferably used in the setting of said minimum and/or maximum set point values of the second PID controller for the mitigation of solvent fog formation. Without cascade the first controller can directly set the valve position, (e.g., a 4 to 40 mA position signal to the actuator on the mixing valve to control the mixed coolant temperature entering the coil in this case). Alternatively, when incorporating the optional second controller (as in cascade) the output of the first controller is expressed in temperature units (e.g. ° C.) and is the set point input to the second controller which has as its output the position signal. The intermediate temperature signal in the cascade arrangement has advantages, for example upper and lower bounds on set point temperature in the second controller may be established based on engineering parameters or physical property behavior in contrast to just rote positioning without the cascade arrangement.

In an optional embodiment for mitigation of fog formation, the temperature of the cooling media exiting the coil is measured and input to a third PID controller with output in communication with the aforementioned flow control device, preferably a variable speed centrifugal pump setting the speed and thereby the flow of cooling media flowing through the coil. The set point temperature input to the third PID controller (an operator input parameter determined as described below) is compared to the measured temperature and the output of the third PID controller adjusts the speed of the pump and hence the flow rate of coolant media to reach the desired target temperature.

Optionally, in an alternative embodiment the output from said exit coolant media temperature measurement is input to a third PID controller configured to calculate by control algorithm the flow set point for the cooling media entering the coil. Accordingly, the output of said third PID controller is in communication with a fourth PID controller which receives the flow set point from said third PID controller in a cascade control arrangement between the two respective PID controllers. The fourth PID controller output sets the speed of the circulating pump to control the measured flow of the cooling medium entering the coil to the input set point of said third PID controller. In this cascade arrangement, the output of the third controller and therefore the set point of the fourth controller is available in flow rate units (such as liters per minute) as an engineering parameter which can be bounded between allowable minimum and/or maximum set point values in accordance with the functional capability of the coolant system. This flow rate parameter can also be translated into other engineering parameters by calculation, including but not limited to cooling duty of the coil according to theoretical or empirical heat and mass balance methods known in the art, or from experimental test measurements. These parameters are optionally used in the setting of said minimum and/or maximum set point values of the fourth PID controller for the mitigation of solvent fog formation.

Thus, the output of the third controller can directly set the speed of the pump by means of, for example, a 4 to 20 mA signal. Alternatively, incorporating the fourth controller allows for an output from the third controller in terms of fluid flow rate (e.g. gpm or lpm) to be input to the fourth controller which sets pump speed by, for example, a 4 to 20 mA signal to bring the measured flow in accordance with the flow set point. The input to the third controller may be set to a value less than or equal to the exit air, e.g., 0 to 12 Celsius degrees below the temperature of the air exiting the coil. Set points for the flow input to the fourth controller can be bounded in terms of flowrate and have engineering/physical significance in optimization of coil performance. The input flowrate set point to the fourth controller is resolved by interaction of the third and fourth controllers in meeting temperature set points of the first and third controllers.

The desired target temperature for the coolant media at the coil exit may be selected to mitigate fog formation according to various film fog formation models found in the literature or by experimentation.

In the example case of condensing NMP from vapor in a given flow rate of air entering the condensing step, experimental results obtained in pilot-scale testing showed that fog formation could be greatly reduced and even eliminated by operating the condensing coils with the coolant media exit temperature held to a value less than the exit air temperature from the coil. In order to accomplish the condition where exit coolant temperature is less than the air exit temperature for a respective coil, first an exit air temperature was selected and coolant media inlet temperature adjusted to reach said exit air temperature. Second, the coolant media exit temperature was measured and the coolant media flow rate adjusted, which alters the temperature rise of the coolant media from coil inlet to exit, to reach a coolant exit temperature from 0 to 12 Celsius degrees less than the coil exit air temperature, most preferably 1 to 5 Celsius degrees less than the coil exit air temperature. The results from setting of the first and second steps are interactive requiring a set of iterative repetitions of these steps until both exit air and exit coolant media temperature conditions are satisfied. Similarly, the process may be carried out beginning with the second step and then proceeding to the first. Preferably the above steps are carried out continuously by PID controller loops as previously described. In addition to the foregoing setting of flow rate and temperatures for a given coil, the design of the coil or set of coils to perform the condensation operation must be selected to match the thermal duty for cooling the sensible heat of air stream plus the heat of phase change while meeting the air exit temperature and coolant media temperature and flow conditions presently described. Common practice for coil design is typically focused on efficient utilization of coolant media from the plant chiller utility while minimizing the physical coil size and pressure drops of air-side flow, typically through rows of finned tubes, and fluid-side flow, typically through multi-pass arrays of tubes. Multi-pass tube arrays are most often manifolded and piped to conduct the fluid media in successive passes countercurrent to the direction of air-side flow. Conventional coil design often results in the coolant exit temperature exceeding the coil air-side exit temperature which results in lower coolant flow rates and pumping costs and a favorable coolant return temperature returning to the plant chiller utility. However, in the case of solvent fog formation in a condenser coil, this higher coolant exit temperature has been associated by test results with conditions resulting in greater fog formation. Consequently, it is advantageous to specify in the duty specifications of the coil that the desired exit temperature of the coolant media is to be within the range of 0 to 12 Celsius degrees, preferably 1 to 5 Celsius degrees, below the specified air exit temperature. This specification typically results in greater design rate of coolant flow as compared to conventional selection for a given cooling duty.

Further, in meeting the described coolant exit temperature condition in condensing operations with large air-side temperature reduction it is often advantageous to divide the cooling duty over two or more coils or stages handling the air flow in series with each coil having separate coolant media flow circuits with controls as previously described for a single coil. This facilities reaching the reduced coolant exit temperature with respect air exit temperature in each coil stage.

Another aspect of coil design to reduce fog formation is the selection of air-side fin spacing and air velocity. It is advantageous in mitigation of fog formation to operate the coil such that channel flow between fins is predominately turbulent as opposed to laminar or transitional. Common fin arrangements may comprise straight parallel fins of thin conductive material such as aluminum, copper or alloy steel evenly spaced to form straight flow channels from the air entry face of the coil to the air exit. In other preferable arrangements fins are formed in a corrugated shape and spaced evenly causing the airflow to follow a serpentine or sinusoidal path between fins increasing turbulence. In conventional coil design for typical duty, fin spacing and air velocity are selected to obtain compact coil size with low pressure drop and efficient utilization of coolant media from the plant chiller utility. By contrast, for fog reduction or elimination higher air velocity and/or greater fin spacing are favorable to reduction or elimination of fog formation. Air velocities greater than 1.5 meters per second are preferred with fin spacing of 3.2 millimeters or greater. Expressed as dimensionless fin Reynolds Number, Re_(L) where Re_(L)=(fin spacing)(air velocity)/(kinematic viscosity), the fin spacing and velocity should be selected such that Re_(L)>400. The result of this velocity and fin spacing tends to result in a larger coil dimension in the flow direction which may require special consideration of the coolant flow rate to meet the duty specification and desired lower coolant exit temperature. In some cases larger diameter for the coil tubes may be needed. Again these design features tend to contradict conventional coil design parameters and should be carefully specified in coil selection where fog formation is to be avoided.

The controller or controllers used in any of the embodiments disclosed herein may have a processing unit and a storage element. The processing unit may be a general purpose computing device such as a microprocessor. Alternatively, it may be a specialized processing device, such as a programmable logic controller (PLC). The storage element may utilize any memory technology, such as RAM, DRAM, ROM, Flash ROM, EEROM, NVRAM, magnetic media, or any other medium suitable to hold computer readable data and instructions. The controller unit may be in electrical communication (e.g., wired, wirelessly) with one or more of the operating units in the system, including one or more of the valves, actuators, sensors, etc. The controller also may be associated with a human machine interface or HMI that displays or otherwise indicates to an operator one or more of the parameters involved in operating the system and/or carrying out the methods described herein. The storage element may contain instructions, which when executed by the processing unit, enable the system to perform the functions described herein. In some embodiments, more than one controller can be used.

The method of any of the forgoing embodiments, alone or in combination, may still further comprise feeding the circulation air at the first volume flow to a pre-condenser upstream said main-condenser with a pre-condensing temperature level below a first temperature level and well above said main condensing temperature; gradually cooling the circulation air to said pre-condensing temperature level in the pre-condenser; and providing the cooled circulation air to an intake of said main condenser. The circulation air may be pre-cooled upstream of said main condenser, and/or may be reheated downstream of said main condenser.

The method of any of the forgoing embodiments, alone or in combination, may still further comprise feeding the off-gas side stream to an at least two-stage air pollution control device; collecting and increasing the concentration of residual condensable fluid in an adsorptive concentrator as a first pollution control stage; and subsequently treating the remaining off gas stream in a second air pollution control device as a second stage further downstream of the first stage to a level of concentration of residual condensable in the air well below a predetermined limit, such as 1 mg/Nm³. The second air pollution control device may be a second adsorptive concentrator or a filtration device, such as an active carbon filter.

The method of any of the forgoing embodiments, alone or in combination, may still further comprise adding humidity to the circulation air feed line.

By operating a single or multi-stage condenser in combination with one or more emission control devices, such as two VOC concentrator wheels in series, the target emission of <1 mg/Nm3 is obtainable. In some embodiments, recovery of VOC solvents results in elimination of nearly all VOC waste products to the environment without requiring thermal combustion products or other secondary pollutants. Valuable solvents such as NMP may be recovered and purified for reuse in the battery electrode manufacturing process in a closed-loop fashion, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments disclosed herein may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting. This disclosure includes the following drawings:

FIG. 1 is a schematic diagram of an exemplary single-stage condenser and downstream emission control apparatus in accordance with certain embodiments;

FIG. 1a is a schematic diagram of an exemplary single-stage condenser and downstream emission control apparatus in accordance with an embodiment including an air-to-air economizer;

FIG. 2 is a schematic diagram of an exemplary multi-stage condenser and downstream emission control apparatus in accordance with certain embodiments;

FIG. 3 is a process diagram of a first exemplary process in accordance with certain embodiments;

FIG. 4 is a process diagram of a second exemplary process in accordance with certain embodiments;

FIG. 5 is a schematic diagram of a direct contact condenser operation in accordance with certain embodiments;

FIG. 6 is a schematic view of a multi-stage adsorption/desorption unit;

FIG. 7 is a schematic diagram of an exemplary single-stage condenser and downstream emission control apparatus with the addition of humidity to the process stream, in accordance with certain embodiments; and.

FIG. 8 is a schematic diagram of an exemplary single-stage condenser and multiple-stage downstream emission control apparatus, in accordance with certain embodiments.

DETAILED DESCRIPTION

A more complete understanding of the components, processes and apparatuses disclosed herein can be obtained by reference to the accompanying drawings. The figures are merely schematic representations based on convenience and the ease of demonstrating the present disclosure, and is, therefore, not intended to indicate relative size and dimensions of the devices or components thereof and/or to define or limit the scope of the exemplary embodiments.

Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawing, and are not intended to define or limit the scope of the disclosure. In the drawing and the following description below, it is to be understood that like numeric designations refer to components of like function.

The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.

As used in the specification, various devices and parts may be described as “comprising” other components. The terms “comprise(s),” “include(s),” “having,” “has,” “can,” “contain(s),” and variants thereof, as used herein, are intended to be open-ended transitional phrases, terms, or words that do not preclude the possibility of additional components.

All ranges disclosed herein are inclusive of the recited endpoint and independently combinable (for example, the range of “from 2 inches to 10 inches” is inclusive of the endpoints, 2 inches and 10 inches, and all the intermediate values).

As used herein, approximating language may be applied to modify any quantitative representation that may vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about” and “substantially,” may not be limited to the precise value specified, in some cases. The modifier “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression “from about 2 to about 4” also discloses the range “from 2 to 4.”

It should be noted that many of the terms used herein are relative terms. For example, the terms “upper” and “lower” are relative to each other in location, i.e. an upper component is located at a higher elevation than a lower component, and should not be construed as requiring a particular orientation or location of the structure. As a further example, the terms “interior”, “exterior”, “inward”, and “outward” are relative to a center, and should not be construed as requiring a particular orientation or location of the structure.

The terms “top” and “bottom” are relative to an absolute reference, i.e. the surface of the earth. Put another way, a top location is always located at a higher elevation than a bottom location, toward the surface of the earth.

The terms “horizontal” and “vertical” are used to indicate direction relative to an absolute reference, i.e. ground level. However, these terms should not be construed to require structures to be absolutely parallel or absolutely perpendicular to each other.

Turning now to FIGS. 1, 1 a and 2, where like numerals indicate like parts, there is shown a condenser 10 having a feed port 12 and an exhaust port 14 spaced from the feed port 12. A portion of the process gas exiting condenser 10 is conducted via stream 30 to pollution control unit 40. In the embodiments shown in FIGS. 1 and 2, the entry of condenser 10 has optionally an air particulate removal filter 18, one or more pre-cooling regions 15, each having a pre-cooling chamber 15 a; one or more cooling or condensing regions or stages 16, downstream of the one or more pre-cooling regions 15, each having a condensing chamber 16 a; a mist coalescing panel (i.e., a demister) 28, and one or more re-heating regions 17, downstream of the one or more cooling or condensing regions 16, each having a re-heating chamber 17 a. Each of the aforementioned chambers is accessible by or permeable for the circulation of air. In certain embodiments, the cooling or condensing region 16 may be a single stage condenser (FIG. 1), or may comprise multiple cooling stages (FIG. 2), each effective to further cool the entering gas stream to a temperature lower than the temperature to which the gas stream was cooled in the cooling stage immediately upstream of it.

In some embodiments, there are two cooling or condensing regions 16, 16′ (FIG. 2). In some embodiments, there are three, four, five or more condensing regions (not shown). The number of condensing regions depends in part on the properties of the VOC's and how gradual the VOC-laden process stream should be cooled to avoid fog formation; i.e., it is at least in part a function of the rate of temperature drop or temperature cooling profile of the VOC's.

In certain embodiments, each cooling or condensing region or stage 16 may include a condensing chamber 16 a having a cooling coil 20 arranged therein, through which a cooling medium may be circulated. Suitable cooling media are not particularly limited, and include water and brines, such as water mixed with propylene and/or ethylene glycol. Each cooling coil 20 may be partially or completely arranged within its respective cooling or condensing region 16. As shown in FIG. 1, preferably the flow of cooling media through the cooling coil 20 is in a counter-flow direction to the flow of process gas through the condensing chamber 16 a. In some embodiments the coolant flow may be configured in a co-flow as depicted by flow streams 116 a and 116 b in FIG. 3 and FIG. 4.

Alternatively, a spray condenser could be used where the coolant is sprayed in the condensation chamber by one or more nozzles to condense the VOC's in the process stream. As shown in FIG. 5, a direct-contact condenser 510 could be arranged in place of condenser 10 of FIGS. 1-4. Solvent-laden airflow 70 from a drying operation is directed to direct-contact condenser vessel 505 after precooling in economizer heat exchanger 101. The cooled stream 70 a is optionally pre-filtered in air filter unit 569 and pressurized by blower 502 and blown into a port in the lower section of vessel 505. The solvent laden air 570 enters the lower chamber of 505 and is directed vertically through flow distributor 516 into a region of media packing 515. Flow distributor 516 includes structural elements to support the weight of the packing and weight of liquid adhering to the packing. Said packing may be selected for optimum surface contact area with the upward-flowing air stream per unit volume and for low pressure drop characteristics. Common packing shapes include Raschig rings, saddles, pall rings or other suitable packing shapes. The packing is generally of corrosion resistant material such as stainless steel, ceramic or polymeric materials. The solvent-laden air stream 570 passes through the packing media and directly contacts cooled condensed solvent flowing downward by gravity and wets the surfaces of the packing. Said cooled condensed solvent is fed to the upper portion of vessel 505 via flow distribution manifold or header 513 and may optionally be further distributed by fluid tray 514 to evenly distribute the cooled solvent evenly across the horizontal cross section at the top of the packing 515. Cooled solvent 538 is used directly as condensing media by direct contact between solvent laden air stream 570 and the wetted surfaces of the packing media in the packing contact region 515. The cooled solvent trickles downward through the packing picking up condensed solvent from the counter flowing air eventually reaching the bottom of packing region 515 and drains through distributor 516 into the lower section of vessel 505 acting as a sump. The level of collected solvent 536 in said sump is maintained by a level sensor and control which directs solvent through conduit 537 to primary storage vessel 38.

The exit gas flow 571 passes through a demister element 528 and a majority of the flow travels through exhaust line 72 a and is reheated in economizer 101 before being conveyed in stream or conduit 72 b to the dryer operation. The split ratio of flow conducted in conduit 30 is preferably in the range of 0.5% to 10% of the flow in exhaust line 572. During unsteady state operating conditions this split ratio can be as high as 20%. The flow in conduit 30 is preferably reheated by heat exchanger 580 to obtain a temperature of flow stream 30 b entering the concentrator 50 between 10° C. and 20° C., most preferably 15° C.

It is to be appreciated that for direct contact condensation, the cooling and condensation surface is in effect the cooled solvent liquid acting as the cooling media as well wetting the surface of the packing and trickling downward countercurrent to the solvent-laden airflow. Having a lower vapor pressure than the solvent laden air, the cooled solvent picks up more solvent from the air as well as increasing in temperature from said air. Therefore in continuous steady state operation energy must be removed from the condensed solvent stream 539 by liquid-liquid heat exchanger 591. The temperature of the cooled solvent fed into the direct condenser must be measured and precisely controlled prior to distributing the cooled solvent into vessel 505 via feed manifold 513. A majority of the collected solvent in 536 in the sump region of 505 is directed through conduit 539 and optionally filtered in liquid filter 518 and further pumped by centrifugal pump 592 and cooled through liquid-liquid heat exchanger 591 and further conveyed to distribution header 513 near the top the tower vessel 505. The split ratio of the flow in conduit 537 is in the range of 5% to 30% of the flow in conduit 539. The flow rate in 539 is measures with a suitable liquid flow meter and controlled to a set point by a controller modulating the speed of motorized pump 592 by variable frequency motor speed control. The temperature of the cooled solvent is measured as it enters the distributor manifold 513 by a suitable temperature element such as an RTD and controlled to a set point temperature, preferably in the range of −10 to 0° C., most preferably −4° C. by the plant utility coolant brine source in exchanger 591. An actuated flow control valve 520 in the coolant flow conduit from brine source is modulated to obtain a desired measured temperature feeding the distribution header 513. Said temperature set point is sufficiently cool such that the vapor pressure of the solvent in the gas phase exiting the packing region 515 results in a concentration level of solvent in stream 571 in the range of 1 to 500 mg/Nm3, preferably 1 mg/Nm³ in the case of NMP.

In certain embodiments, the pre-cooling region 15 and the re-heating region 17 may be brought and/or maintained at their respective operating temperatures with a pre-cooling heat exchanger and a re-heating heat exchanger as in FIGS. 1, 2, 3, and 4. In certain embodiments, these heat exchangers are a closed loop heat exchange recirculation system 35 which may function as an economizer. The recirculation system 35 may include a first coil 35 a at least partially arranged inside the pre-cooling chamber 15 a and a second coil 35 b at least partially arranged inside the pre-heating chamber 17 a. The loop of the recirculation system 35 may contain a suitable heat exchange medium such as water or brine to transfer heat to and/or from the process gas in the pre-cooling and pre-heating regions. An air-to-air economizer heat exchanger 101 also could be used as shown in FIGS. 1 a, 5, 7, and 8. Exhaust flow 70 from the drying operation is precooled in economizer 101 by the cooler temperature return air flow in exhaust line 72 a. The economizer 101 is selected to provide thermal exchange effectiveness in the range of 40 to 60% resulting in temperature of flow stream 70 a at the proper pre-cooled temperature value in the range of 80 to 100° C. Accordingly, the exhaust return air 72 a is reheated with energy from exhaust flow 70 to a desired temperature in the range of 40 to 60° C. in the economizer 101. The temperature of the heat exchange medium optionally may be regulated with the aid of a further heat exchanger 291 as shown in FIG. 3. In certain embodiments, the temperature of the coolant in the loop of the recirculation system 35 is higher than the temperature of the coolant in the coil or coils in the condensation region 16. Excess thermal energy may be removed to control the temperature of the coolant in the loop of the recirculation system 35 by circulating a portion of the coolant from the loop of the recirculation system 35 via pump 292 through a liquid-liquid heat exchanger 291, in a split ratio range of 10 to 50% of the flow through 35. The portion of fluid thus cooled is returned in conduit 293 to 35 before entering 35 b.

In certain embodiments, the pre-cooling region 15 and the condensing region 16 have a common housing. In certain embodiments, the condensing region 16 and the re-heating region 17 have a common housing. In certain embodiments, the pre-cooling region 15, the condensing region 16 and the pre-heating region 17 have a common housing.

In some embodiments, a pre-cooling first coil 35 a may be arranged upstream of the pre-cooling region to reduce the temperature of the circulation air exhaust flow 70. In some embodiments, a re-heating second coil 35 b may be arranged downstream of the condensation region 16. In some embodiments, both coils may be so arranged. The pre-cooling coil and the re-heating coil may be thermally coupled by the exchange of a heat transfer medium such as water, brine or suitable thermal fluid.

In certain embodiments, a side-stream off gas extraction line 30 may be provided in fluid communication with the condensation region 16. Where multiple condensing regions are provided, the side-stream off gas extraction line is preferably in direct fluid communication with the furthest downstream condensing chamber 16 a′; i.e., the chamber immediately upstream of the re-heating region 17. The side-stream gas extraction line 30 may be configured to communicate with a downstream unit operation, such as one or more VOC concentrators and/or one or more emission control units as discussed in greater detail below. A portion of side-stream off-gas may also be extracted following the re-heating region through line or conduit 29 and added to the side-stream flowing in extraction line 30. Control system 31 comprised of a temperature sensor and controller positioned to measure the mix temperature of flows from extraction lines 29 and 30 and modulate actuated flow dampers in flow lines 29 and 30. Thus an optimum temperature in the range of 10 to 18° C., preferably 15° C. may be obtained and controlled by closed-loop control of flow proportioning as the flow stream 30 b enters the concentrator 50. A circulation air exhaust line 72 may be provided in fluid communication with the exhaust port 14 of the condenser 10. The circulation air exhaust line 72 may be configured to connect to a feed circulation air duct of one or more industrial dryers (FIG. 3), for example. The split flow proportion flowing in stream or conduit 30 a is preferably in the range of 0.5% to 10% of the flow in exhaust line 72. During unsteady state operating conditions this split ratio can be as high as 20%.

In some embodiments, the feed port 12 of the condenser 10 is in fluid communication with, or is adapted or configured to be in fluid communication with, the exhaust from one or more industrial dryers 200 (FIG. 3), such as by a circulation air feed line 70. That is, the source of the process gas to the condenser 10 may be the exhaust from one or more industrial dryers, such as one or more dryers drying coating on a web. For example, in battery electrode manufacturing, a wet process is used to apply battery electrode components to a current collector web of copper or aluminum foil, for example. Typically a polymer binder, graphite, and an active material are mixed with a solvent, or water, and applied to a web. The solvent or water is driven off in one or more dryers to produce a dry battery electrode material for use in a battery cell. The exhaust from such a dryer or dryers is a suitable feed stream to the condenser 10.

In some embodiments, the exhaust port 14 of the condenser 10 is in fluid communication, or is adapted or configured to be in fluid communication, with a recirculation feed to one or more industrial dryers 200 (FIG. 3), such as by a circulation air exhaust line 72. A suitable driving force, such as a suction fan arranged in the recirculation line 72, may be used to drive the flow of process gas into and through the condenser 10.

The dryer or dryers 200 may be a conventional convection air web dryer such as where hot air is used to dry a coating on a web travelling through the dryer. For example, air bars or nozzles may be used to direct a jet of air to impinge on the surface of a material to carry out heat and/or mass transfer functions. Convection air drying of battery electrode materials is typically carried out with drying air temperatures in the range of 80 to 160° C. and air nozzle convection heat transfer coefficients to the web from 30 to 100 watts/m²° C. per side of web. As is known to those skilled in the art, a plurality of air nozzles may be arranged in an array or multiple arrays to direct air impingement over a large surface of a material in web form, either on one side of the web, or both sides simultaneously. In some embodiments, the heat transfer to the web may be enhanced by addition of infrared emitters or emitting surfaces. Optimum temperatures for said emitting surfaces are in the range of 260 to 425° C. Web materials commonly processed in this manner include paper, plastic film, metal foils, woven and non-woven fabrics and mats, and porous membrane materials. Flotation air bars are a type of air nozzle used in industrial dryers and ovens to floatingly support and convey a continuous web to be processed by thermal treatment, which may include any combination of drying, heating, curing or cooling of the web. A coating is applied to the surface of the web or a volatile material is present within the base web material which must be dried and/or heated to a particular temperature so as to facilitate thermal curing of a polymer material in the coating. In many processes the volatilized materials within the web or coating after being liberated from the web surface are carried away from that surface by the spent nozzle air and conducted by an air handling system to an exhaust. This exhaust is then directed into the condenser 10 in accordance with certain embodiments.

Alternatively, the dryer may be a so-called “inert” dryer, in which the dryer interior contains an inerting gas such as nitrogen in order to limit the oxygen content (e.g., to 2% or less) of the dryer atmosphere to, for example, reduce the possibility of explosion. Said nitrogen may be vaporized from liquid nitrogen storage tanks or produced continuously from nitrogen generator systems having membrane separators or pressure swing adsorption modules.

Recovered condensate may be removed from the condenser 10 and stored in a suitable container or storage vessel 38. Each condensing stage 16 and 16 a′ and coalescing demister 28 may be configured to drain condensed solvent by gravity to sump basin 36. Drain line 37 may include an air lock (s-trap or other suitable device) to prevent passage of air from condenser 10 to container 38 and vice versa.

In certain embodiments, the pre-cooling stage(s) 15 of the condenser 10 functions as a pre-condenser; it is operated at a suitable temperature (and/or pressure) such that little or no VOC's condense as the process gas flows through the one or more pre-cooling chambers 15 a.

Most preferably the cooling coil 20 in the condensing region 16 is operated so that the gas exiting condensing chamber 16 a has a main temperature of 0° C. or less. In some embodiments, where multiple condensing regions 16 are used, the cooling coil arranged in the stage furthest downstream is operated such that the chamber in which it is arranged has an exit temperature of 0° C. or less, such as at 0° C., −1° C., −2° C., −3° C., −4° C., −5° C., or down to −20° C. in cases where the anti-freeze behavior due to solvent in the solvent-water mix is favorable against freezing and ice formation in the condensing coil. For such low temperature condensing it is especially important the cooling coil(s) be arranged in the one or more upstream condenser stages and operated such that their respective chambers have higher temperatures such that only a small portion of the VOC's in the upstream stages are condensed.

It is a goal of each condenser coil design and operating condition to maximize condensation with wetting of solvent on the tube and finned surfaces to promote capture and gravity drainage. Rapid cooling of the solvent laden air below equilibrium vapor pressure of the solvent promotes the formation of tiny droplets within the bulk air stream as it travels between condensing coil tube and finned surface. After nucleation, said tiny droplets tend to remain very small with negligible further condensation and growth owing to the Kelvin effect. Accordingly, in order to reduce or eliminate deleterious fogging, in certain embodiments the rate of cooling is carefully controlled so as not to cool the VOC-laden stream too rapidly. Fogging, or the formation of very small droplets (generally on the order of 1 micron or less) of liquid, is problematic in that it involves the formation of tiny droplets that tend to become entrained in the air flow, thus hindering their removal or recovery. Indeed a significant fraction of such aerosol droplets pass through demister panels as well as pass through the core of the condenser coil. As a result, fog or aerosol formation results in the undesirable loss of product. In embodiments where the VOC includes NMP such as in lithium battery electrode production, for example, these tiny droplets of NMP end up being returned to the dryer(s), which is highly undesirable. Accordingly, the pre-cooling region(s) 15 and the condensation region(s) 16 should be operated such that gradual cooling of the process stream takes place so as to avoid fog formation. In many cases gradual cooling is best carried out by dividing the cooling duty over two or more coils or stages handling the air flow in series, with each coil having separate coolant media flow circuits with respective controls.

For purposes of cooling rate characterization and quantification, the residence time of the solvent laden air while within the core of a particular condensing coil (i.e. while undergoing cooling) is made on a superficial volume basis. That is, the volume space occupied by the tubes and fins within the overall dimensions of the core is ignored in calculating the superficial residence time. The specific volumetric airflow Q through the coil is expressed in normal cubic meters per unit time. The coil face area and depth in the flow direction are used to calculate the volume V in cubic meters. Therefore the superficial residence time in the core is determined as t=V/Q. Further, the temperature drop DT in Centigrade degrees (note this parameter is a temperature difference value, not an absolute temperature value) of the air and solvent driven by the cooling coil may be measured directly in operation or calculated from supplier sizing data in the design phase. Finally the rate of cooling R may be expressed as R=DT/t. For example, a cooling coil core has a face area of 4.5 square meters and a flow path depth of 0.3 meters. The specific volumetric airflow is 25,000 Nm³/h. Therefore the residence time may be calculated as t=(4.5×0.3)m³/25,000 m³/hr=5.4×10⁻³ hr. Converting to milliseconds t=5.4×10⁻⁵ hr×3,600,000 millisec/hr=194 milliseconds. If the air temperature entering the coil is 56° C. and the exiting temperature is 28° C. the cooling rate R=DT/t=(56° C.−28° C.)/194=0.144 C degrees per millisecond.

For NMP capture with minimal fog formation in the condensing coil regions where condensation of solvent is occurring while the gas flow travels the distance between the condensing coil tubes and finned surfaces, the rate of cooling should be less than 0.30 C degrees per millisecond, preferably between 0.15 and 0.22 C degrees per millisecond. Typical design criteria for organic solvents miscible with water for minimal fog formation in the condensing coil regions where condensation of solvent is occurring while the gas flow travels the distance between the condensing coil tubes and finned surfaces the rate of cooling should be less than 0.3 C degrees per millisecond, preferably between 0.1 and 0.2 C degrees per millisecond. In most cases the acceptable maximum cooling rates for a particular solvent must be determined by experiment.

In operation the method of precise control of the cooling rate may be illustrated in reference to FIG. 2 for the exemplary case of chamber 16 a and coil 20. This same method is applicable to a plurality of cooling coils represented as 22 within condensing chamber 16′. It is to be understood that the circulating flow loop and controls described below shall apply to additional coils in like manner. All such flow loops shall include and the temperature control circuits and hardware shown for chamber 16 a and coil 20.

The flow and temperature of the cooling medium 20 a entering the coil 20 are preferably measured with a suitable flow meter device and temperature sensor such as a resistance temperature detector (RTD) respectively. The circulating flow is driven by a fluid pump 21 a in communication with the fluid entry connection of the cooling coil 20. Further, the temperature of the air entering the cooling coil is preferably measured with an array of one or more temperature sensors 21 d spaced across the cross-section of the entry face of the coil and the temperature of the air exiting the cooling coil is preferably measured with an array of one or more temperature sensors with controller 21 c spaced across the cross-section of the exit face of the coil. Said air temperature sensors may be RTD's or thermocouples. The temperature of the entering cooling media is measured and controlled to a pre-determined set point by a suitable sensor with PID controller 21 b in control communication with a valve and actuator 21 positioned in the conduit 20 c from the chilled brine source, typically a water-cooled or air-cooled centrifugal chiller. An actuated three-way flow path valve 21 acting as a flow proportioning valve allows fresh cooling media from the cooling media source to enter the circulating flow path through the coil while heated cooling media in fluid communication with the cooling coil return connection 20 b is discharged back to the cooling media source. In a preferred control arrangement, the temperature of the air exiting the cooling coil is controlled to a pre-determined set point by a suitable PID controller 21 c in control communication with a valve and actuator 21 acting as a flow proportioning valve positioned in the conduit from the chilled brine source. In a most preferred embodiment, the output from said exit air temperature measurement and controller 21 c is configured to calculate by control algorithm and transmit in a cascade control arrangement said temperature set point to the entering cooling media control loop PID controller with temperature sensor 21 b. The pre-determined set point for the air exiting the cooling coil is selected by the operator to reach the desired target concentration of solvent or aqueous humidity of the air exiting the coil as determined from theoretical and or empirical vapor-liquid equilibrium (VLE) equations, such as the Antoine Equation and Raoult's Law or engineering simulation model tools such as ChemCad, ASPEN, or other theoretical or empirical methods known in the art, or preferably from experimental test measurements.

The specified test result conditions discussed in the following example below relate to a third coil, i.e. the final condensing coil in a series of four coils within a condenser system 10 according to FIG. 2. The first coil is a pre-cooling coil where no condensing was performed, the second an initial condensing coil, the third a final condensing coil, and the fourth a reheat coil. It is to be understood the general methodology and elements of condensing region 16 applies similarly to subsequent condensing region 16′ and any subsequent condensing regions to be applied in condenser systems with additional condensing regions. For this example in the case of condensing NMP from dry air, a set point of the exit air temperature to controller 21 c for a vapor pressure in dry air equating to 45 ppm by volume is approximately −2° C. based on empirical correlation. Naturally there is some uncertainty around such empirical predictions, consequently the actual requirements are best verified experimentally in operation in a pilot or prototype condenser unit. In some cases the set point exit air temperature is set lower than that determined by VLE to accommodate for uncertainty. Settings from 0 to 5 Celsius degrees lower than VLE predictions may be needed to meet the target solvent concentration in the coil exit air.

In a preferred embodiment shown in FIG. 2 the output from exit coolant media temperature sensor and PID controller 20 e is input to the drive control of pump motor 20 f which accordingly sets the speed of the circulating pump to modulate the flow of the cooling medium entering the coil to the input set point of PID controller 20 e. The set point of controller 20 e is determined from fog mitigation analysis.

The impact of potential aerosol fog formation is also to be considered in determining the set point of the coolant media entry and exit temperatures and flowrate. Fog droplets that cannot be captured by condenser demisting elements, although phase change has occurred, will remain in the exit air stream and re-evaporate after the condenser exit gas is reheated for use in the dryer or other downstream operations. In experiments condensing NMP, exit concentration values were measured by flame ionization detector (FID) instruments in sampled air after being drawn through a PTFE membrane filter to remove sub-micron mist particles and subsequently heated were found to match vapor-liquid equilibrium calculations. Additionally, air samples were drawn and measured without filtering and measured after sample heating. The unfiltered values ranged up to 80 ppm while the filtered values were in the range of 45 to 50 ppm. The difference in the two measurements represents the magnitude of fog formation. An additional set of parameters was investigated by reviewing the technical literature and by experimentation with the goal reducing or eliminating the magnitude of fog formation. Experimental results showed that the preferred set point exit temperature of the coolant media to controller 20 e is to be within the range of 0 to 12 Celsius degrees, preferably 1 to 5 Celsius degrees, below the specified air exit temperature. In the previous example where coil exit air temperature is set to −2° C., the air velocity between coil fins was 2.1 meters per second the coolant media entry temperature was set to −7° C. with coolant flow rate adjusted to obtain a coolant media exit temperature of −4° C., that is 2 Celsius degree below the coil exit air temperature. The filtered exit concentration of NMP in the coil exit air was 45±5 ppm while the unfiltered exit concentration of NMP was 48±5 ppm, a difference of 3 ppm NMP representing a low rate of fog formation.

In an alternate cascade arrangement, the output of controller 20 e is communicated as the input set point of controller 20 h setting the flow rate set point for the measured flow at flow sensor 20 g. The output of Controller 20 h is input to the drive control of pump motor 20 f which accordingly sets the speed of the circulating pump to modulate the flow of the cooling medium entering the coil to the input set point of PID controller 20 h. In this alternate arrangement, the output of controller 20 e is available in flow rate units (such as liters per minute) as an engineering parameter which can be bounded between allowable minimum and/or maximum set point values in accordance with the functional specifications for pumping capacity of the coolant system. This flow rate parameter can also be translated into other engineering parameters by calculation, including but not limited to cooling duty of the coil according to theoretical or empirical heat and mass balance methods known in the art, or from experimental test measurements. These parameters are optionally used in the setting of said minimum and/or maximum set point values of PID controller 20 h for the mitigation of solvent fog formation.

In certain embodiments, once the process gas flow from the condensing region 16 has reached 0° C. or less and VOC condensation is complete or substantially complete, the gas flow is divided into a relatively high volume recirculation gas flow (e.g., line 72) and a relatively low volume off-gas side stream (e.g., extraction line 30). In the example shown in FIG. 3, those volumes are split by a split-ratio of approximately 2%.

In certain embodiments, the blend of off-gas side stream extraction lines 29 and 30 combining into 30 a is adapted or configured to be placed in fluid communication with one or more downstream emission control operations 40. For example, the emission control operation may include at least one adsorptive concentrator with a gas exhaust and a desorption exhaust, such as one or more VOC adsorptive concentrators 50 used to concentrate the VOC's extracted by the condenser 10. The type of VOC concentrator(s) used is not particularly limited, and may be rotation type gas adsorpton concentration devices as sold by SG America, Inc. or Nichias Corporation of Japan. For example, honeycomb rotors may be used to support the VOC adsorbent media, and the rotor may be divided into at least an adsorption zone and a desorption zone. The solvent-laden air to be processed is passed through the adsorption zone, where the VOC's are adsorbed by the adsorbent media (e.g., zeolite). The VOC's can then be desorbed, such as by passing heated air through the desorption zone. In some cases, such concentrators may include a rotor wheel having an adsorbent substrate such as a hydrophobic zeolite, or a combination of adsorbents. The rotor wheel may be rotated continuously, and the airstream passes through the rotor wheel concentrator where VOC's are stripped from the air and adsorbed onto the adsorbent substrate. The majority of this now clean air may then be exhausted to atmosphere. A small portion of the air stream may be heated to an elevated temperature to be used as desorption air. Continuous rotation of the wheel transitions this portion of the wheel now with adsorbed solvent to a desorption region, where the VOC's are heated, desorbed into said heated desorption air stream and subsequently collected from the desorption air in a subsequent solvent removal device such as a condenser coil. Purge air may be used to cool the adsorbent media, and the now heated purge air can be recycled to the desorption region. Thus, the one or more adsorptive concentrator 50 may have a gas exhaust and a desorption exhaust and function as an emission or pollution control stage.

In certain embodiments, a single VOC concentrator 50 may be used. In other embodiments, two or more VOC concentrators 50 and 50′ may be used, arranged in series. In some embodiments, one or more further emission or pollution control stages 52 may be used downstream of the one or more VOC concentrators 50, such as one or more filtration devices (e.g., activated carbon based filtration devices), absorptive concentrators, thermal oxidizers (e.g., regenerative thermal oxidizers), catalytic oxidizers and/or biofilters.

In some embodiments, a two-stage air pollution control unit 500 as shown in FIG. 6 may be arranged downstream of the condenser 10, such as the pollution control unit disclosed in JP 2011-031159. The unit includes an organic solvent recovering device wherein an adsorbing body comprising an adsorbing element containing an adsorbent is constituted of at least an adsorbing region, a regeneration region and a cooling region. The organic solvent-containing gas (e.g., the gas in the side-stream 30 from the condenser 10) is continuously supplied to the adsorbing region of the adsorbing body, the adsorbing element adsorbing the organic solvent in the adsorbing region, and is sent to the regeneration region to desorb the adsorbed organic solvent from the adsorbing element by a heated gas. The organic solvent-containing gas is again adsorbed in the adsorbing region by the regenerated adsorbing element, and a condensing part for recovering the organic solvent desorbed in the regeneration region is provided. A backup treatment device capable of performing continuous adsorbing and desorbing treatment of the organic solvent-containing treated gas passed through the adsorbing region without being adsorbed is provided, and is constituted so that the continuous solvent adsorbing and desorbing treatment is performed in the adsorbing region and the regeneration region by the adsorbing element formed into a columnar or cylindrical shape.

FIGS. 3 and 4 illustrate exemplary embodiments of the operation of a condenser and emission control apparatus. In an exemplary embodiment as shown in FIG. 3, an exhaust gas stream 70 containing VOC's, such as NMP, that is exiting one or more dryers 200 may have a temperature generally between about 80-130° C., more typically between about 120-130° C., and may have a solvent concentration generally between about 500-3000 ppmV, more typically between about 1800-2500 ppmV. In the embodiment shown, the concentration of solvent in the exhaust stream is 1800 ppmV and the temperature is 130° C. A driving force such as a fan 201 may be used to cause the exhaust stream to enter the feed port 12 of a condenser 10. Alternatively or in addition, a suction fan 202 may be placed downstream of the condenser exhaust port 14.

Consistent with the objective of gradually reducing the temperature of the exhaust gas stream, it first enters a pre-cooling region 15 of the condenser 10, the region 15 having a coil 35 a with a cooling medium having a temperature sufficient to lower the temperature of the gas stream to about 83° C. The residence time of the gas stream in the pre-cooling region should be sufficient to allow the temperature of the stream to cool to the desired value, such as 83° C. Since no condensation is occurring in this stage, cooling rates in the range of 0.6 C degree per millisecond or more are acceptable. Preferably the conditions in the pre-cooling region 15 are such that little or no solvent condensation occurs. The gas then flows into a first cooling or condensing stage 16 a, where it is cooled by a coil 116 a containing a cooling medium having a temperature sufficient (e. g., 18° C.) to lower the temperature of the gas stream to 29° C. In this stage most of the heat exchange is sensible heat and although little condensation of solvent occurs in this stage the cooling rate is critical in this early stage of condensing solvent. Cooling rates not to exceed the range of 0.15 to 0.30 C degree per millisecond are preferred. The cooled gas then flows into a second cooling or condensing region or stage 16′, or main condensing stage, equipped with a coil 116 b containing a cooling medium having a temperature sufficient (e.g., −10° C.) to lower the temperature of the gas stream to equal to or less than 0° C.; e.g., −2° C., where significant phase change occurs, and all or substantially all of the remaining solvent condenses. Again, in this stage the cooling rate is critical as in this early stage of condensing solvent. Cooling rates not to exceed the range of 0.15 to 0.30 C degree per millisecond are preferred. This gradual cooling of the gas stream helps minimize or prevent deleterious fog formation.

A major portion of the gas then flows into the re-heating region 17, which in the embodiment shown has a coil 35 b containing a cooling medium having a temperature sufficient to raise the temperature of the gas stream to 45° C., after which it is recirculated to the one or more dryers 200 via exhaust line 72. Ambient air may be added to the exhaust line 72 as shown at 73.

A minor portion (e.g., by mass flow balance equivalent to the flow of gas entering into web slots in the one or more dryers) of the gas flow is extracted from the air exiting the last cooling or condensing region or stage 16′ via a side-stream off gas extraction line 30. In the example shown in FIG. 4, the volumes of the re-circulation air stream 72 and the off gas side-stream 30 are split by a split-ratio of approximately 1.1%. It is preferred that this extraction be carried out upstream of the pre-heating region 17, since the relatively low temperature of this extracted stream tempers air stream 30 a and raises the efficiency of a downstream concentrator. A portion of the gas in the re-heating region 17 may be extracted in conduit 29 and mixed with the gas in the side-stream off gas extraction line 30 extracted from the second cooling or condensing region 16′. The amount of gas that has been re-heated in region 17 that is mixed with the gas in the side-stream off gas extraction line can be blended with temperature controller and damper control system 31 to regulate the temperature of the gas entering a downstream concentrator (or other downstream emission control apparatus) to optimize the performance of the downstream unit, such as regulating the gas in the side-stream off gas extraction line 30 a to a temperature of about 10-20° C., preferably about 15° C.

Thus, the condenser apparatus 10 can be used to produce a feed stream to a downstream emission control unit or units, such as one or more VOC concentrators, the feed stream being at an optimum temperature for the performance of the emission control unit or units. Off-gas side stream flows extracted from a plurality of condenser apparatuses equivalent to condenser apparatus 10 may be preferably combined in common duct 700 and fed to a single VOC polishing concentrator system as shown in in FIGS. 3 and 4. This allows for better capacity matching of the side stream flows from a large operation having for instance 8 or more condensing apparatuses. Combination of the off-gas side stream flows results in improved economy of scale for the VOC polishing operation.

The gas in the side-stream off gas extraction line enters a first VOC polishing concentrator 50 which contains an adsorbent such as a zeolite or carbon. This first concentrator 50 typically removes from about 90-99% of the VOC's (e.g., NMP) from the gas stream. The adsorbed VOCs may be then desorbed by reheating, and may be directed to a cooling or desorbant condenser 80 or the like and recirculated back to the inlet stream to the first VOC polishing concentrator as shown. VOC condensate may be collected from the condenser 80 via line 81 and stored or recycled to the coating process.

In the embodiment shown in FIG. 3, the concentrated gas stream next enters a second VOC polishing concentrator 50′, which also typically removes from about 90-99% of the VOC's (e.g., NMP) remaining in the stream. In certain embodiments, the second VOC polishing concentrator 50′ is identical to the first VOC polishing concentrator 50. The collected air stream may be condensed in the condenser 80 as shown.

A bypass line 85 may be provided to allow interruption of the concentrator units, the bypass line 85 directing the flow (e.g., with the aid of fan 88) to one or more back-up carbon filters 87, for example, so that the content of the VOC's exhausted to atmosphere does not exceed regulatory limits (e.g., <1 mg/Nm³).

An emergency purge system 90 is provided, which includes purge line 91 having a damper 92 that directs the flow of gas from the condenser 10 to one or more emission control units, such as one or more carbon filters 95.

NMP can be recovered from the condenser 10 via line 117 and stored as shown.

FIG. 4, where like numerals indicate like parts to those described above, illustrates a similar process with only a single VOC polishing concentrator 50 being employed. In this embodiment, two carbon filters 87, 87′ are arranged downstream of the single VOC polishing concentrator 50 in order to achieve the target VOC emission concentration of <1 mg/Nm³. This embodiment also eliminates the emergency purge system 90, instead a purge fan 190 is fluidly connected to the exhaust line 72 as shown to provide an emergency purge.

Turning to FIG. 5, as in the case of coil condensers 10, avoidance of fog formation in direct contact condensers 510 depends on gradual cooling of the solvent-laden air stream in the condensing region, that is within the height of packing 515. Cooling rate is first considered in the design selection of the volume of packing region 515. Condenser vessel 505 is preferably circular in cross-section. Alternatively it may be of square or rectangular cross-section in plan view in order to accommodate site layout requirements. Similar to the case of coil type condensers, the volume is determined on a superficial volume basis. That is, the volume space occupied by the packing within the overall dimensions of the packing region 515 is ignored in calculating the superficial residence time. The specific volumetric airflow Q through the coil is expressed in normal cubic meters per unit time. The packing face area and depth in the flow direction are used to calculate the volume V in cubic meters. Therefore the superficial residence time in the core is determined as t=V/Q. Further, the temperature drop DT in Centigrade degrees (note this parameter is a temperature difference value, not an absolute temperature value) of the air and solvent driven by the cooling coil may be measured directly in operation or calculated from supplier sizing data in the design phase. Finally the rate of cooling R may be expressed as R=DT/t. For example, a direct-contact condenser vessel has a face area of 4.0 square meters and a packing height of 1.5 meters. The specific volumetric airflow is 25,000 Nm³/h. Therefore the residence time may be calculated as t=(4.0×1.5)m³/25,000 m³/hr=2.4×10⁻⁴ hr. Converting to milliseconds t=2.4×10⁻⁴ hr×3,600,000 millisec/hr=864 milliseconds. If the air temperature entering the packing is 83° C. and the exiting temperature is −4° C. the cooling rate R=DT/t=(83° C.−(−4)° C.)/864=0.1 C degrees per millisecond.

For NMP capture with minimal fog formation in the condensing packing regions where condensation of solvent is occurring while the gas flow travels the distance within the packing, the rate of cooling should be less than 0.20 C degrees per millisecond, preferably between 0.07 and 0.15 C degrees per millisecond. Typical design criteria for organic solvents such as DMAc with water vapor for minimal fog formation in the condensing coil regions where condensation of solvent is occurring while the gas flow travels the distance within the packing the rate of cooling should be less than 0.15 C degrees per millisecond, preferably between 0.05 and 0.12 C degrees per millisecond. In most cases the acceptable maximum cooling rates for a particular solvent must be determined by experiment.

In operation the method of precise control of the cooling rate may be illustrated in reference to FIG. 5 for exemplary case of packing 515. The flow meter 524 and temperature sensor 521 monitor flow rate and temperature of the cooled solvent medium entering the manifold 513. The circulating flow 539 is driven by a fluid pump 592 in communication with the fluid entry connection of the cooling coil 591. Further, the temperature of the air entering the packing 515 is preferably measured with an array of one or more temperature sensors 522 spaced across the cross-section of the entry face of packing, and the temperature of the air exiting the demister 528 is preferably measured with an array of one or more temperature sensors 523 spaced across the cross-section of the exit face of the demister. Said air temperature sensors may be RTD's or thermocouples. The temperature of the entering cooled solvent media is measured and controlled to a pre-determined set point by a suitable sensor and PID controller 521 in control communication with a valve and actuator 520 positioned in the conduit from the chilled brine source, typically a water-cooled or air-cooled centrifugal chiller. The measured air temperature drop of the air entering the packing and the air 571 exiting the condensing tower is controlled to a pre-determined set point by a second suitable PID controller 523 in control communication with the variable frequency motor control driving pump 592. In this manner the target rate of temperature drop in the condensing unit 510 is controlled to 0.15 C degrees per millisecond for NMP for example.

FIG. 6 illustrates a different type of adsorption device to replace the single VOC polishing concentrator in FIG. 4. The main adsorption takes place on the path A1 through the first segment of a carousel-type adsorption concentrator 610. The desorption is done with reference to JP2011031159A. A main desorption cycle is performed by the path B2 with a condenser 620 and a heating coil 625 implemented. Additionally the FIG. 6 shows a closed-loop auxiliary adsorption-cooling-desorption circle with the path ‘“A2” (adsorption)—“C” (cooling)—heating—“B1” (desorbing)—cooling’. This configuration has the advantage that the two circles can be operated under inert conditions (if necessary) and can lead higher desorption concentrations in the main desorption circle. As described in FIG. 4 additional carbon filter can be implemented before the gas is released to the stack. The descripted process is also possible with a disc-type concentrator.

The foregoing discussion does not include consideration of condensation of an organic solvent such as NMP with the co-presence of high humidity (e.g., water) in the gas (typically air) stream. In the previous embodiments discussed, the humidity in the solvent laden gas stream is sufficiently low such that vapor-liquid equilibrium conditions do not result in condensation of water along with the solvent. For example, if the dew point humidity level (water vapor) of the gas flow into the condenser is lower than the condensing coil surface temperature, very little if any water will be condensed. On the other hand, if the dew point humidity is above the condensing coil surface temperature, both water and solvent may be condensed, depending on factors including vapor-liquid thermodynamics, contact time, and droplet nucleation behavior. Such condensation is often unpredictable based on thermodynamic properties alone; dynamic experiments are often required and may lead to discovering unexpected results in terms of capturing the organic solvent in the condensed liquid.

In the case of condensing an organic solvent that is miscible in water such as NMP, particularly at temperatures below the aqueous dew point of the humid gas stream, the vapor pressure of the solvent is typically reduced with increasing water content in the incoming air, resulting in reduced solvent vapor pressures exiting the condenser. Thus, in comparison to the case with very low aqueous humidity (condenser temperature above the dew point of the water vapor phase), the exiting concentration of organic solvent is lower when water also condenses along with the organic solvent. In certain cases it is advantageous to reduce the organic solvent concentration in the exit gas stream by adding water vapor to the incoming gas stream. This may allow the final condensing temperature, that is the exit air temperature of the last condensing coil, to be set at higher temperatures due to a shift in VLE behavior. In the dry air conditions of battery electrode processing, comparable concentrations of NMP in the exit air can be obtained with condensing coil exit air temperatures in the range of 0 to 5° C. with water injection compared to air exit temperatures in the range of −5 to −2° C. without injection. This may be used to advantage in the operating and capital cost of plant chiller utilities.

Thus in some cases, it may be advantageous to adjust aqueous humidity conditions of the gas going to the condenser in order to obtain desirable exit concentrations of solvent and/or water in the gas exiting the condenser. For instance, instead of reducing the condenser temperature, even to temperatures below the freezing point of water (as discussed in previous embodiments) in order to reduce the solvent concentration, in certain cases the aqueous humidity of the gas entering the condenser may be increased, such as by introducing water (e.g., by spray or injection), steam or gas containing a high water content, vaporizing water by spraying into said air stream, or heating a source of water such as a container (e.g., a pan or the like) of water in the duct preceding the condenser. This added water vapor may promote condensation of solvent and water while at higher condenser operating temperatures in the condenser. This may result not only in reduced exit concentrations of solvent, but also may increase the energy efficiency of the condensing operation step in terms of reducing refrigeration electrical power load.

FIG. 7 shows an exemplary embodiment of steam injection for adding aqueous humidity according to the forgoing description. Condensing effectiveness of organic solvent species such as NMP on condensing coil 20 may be enhanced by co-condensing an amount of water, which advantageously lowers the concentration of solvent leaving condenser 16 in streams 30 and 72 a. In the embodiment shown, gas temperature sensor with controller 21 c is located at the exit of the condensing chamber 16 a. If more than one condensing coil is present as described in optional embodiments having a plurality of condensing coil stages as previously described, sensor with controller 21 c may be similarly located at the exit of each condensing coil. Sensor with controller 21 c provides a temperature value for the condensing operation representing the saturated vapor concentration in terms of aqueous dew point temperature exiting the condenser. Aqueous dew point sensor 301 provides a dew point temperature value for the entering process gas stream 70 a representing the water vapor concentration entering the condenser 10. In the embodiment shown, pressurized steam is introduced such as by an injector 303 into the duct conveying the gas stream 70 a to raise the aqueous humidity of said stream 70 a before it enters the condenser 10. The desired dew point at sensor 301 is set to a value higher than the condensing temperature sensed by sensor with controller 21 c in a closed loop controller 302, which then causes actuated injector valve 306 to open, adding sufficient water vapor via injector 303 to promote condensation of water vapor in the condensing chamber 16 a. The discharge of injector 303 into stream 70 a may be comprised of a pipe or array of pipes with drilled orifices to discharge said steam, or preferably an injector array having one or more pipes with a plurality of nozzles distributed along each pipe length such as available from Armstrong International, or other suitable steam dispersion element.

Alternatively, instead of measurement of aqueous dew points in streams 70 a and 72 a to control steam injection at injector 303, steam injection may be continuously controlled by alternate closed loop control action of controller 302 by determination of the mole fraction of water in the gas stream 72 a. Optional solvent concentration sensor or analyzer 304 provides a value of the concentration of organic solvent in the gas stream 72 a exiting the condensing unit 10. Suitable gas phase solvent concentration analyzers may be selected from available instruments including flame ionization detectors (FID), Fourier Transform Infrared spectroscopy (FTIR), mass spectroscopy, or other suitable instruments. Optional aqueous dew point sensor or analyzer 305 provides a value of the concentration of aqueous humidity in the gas stream 72 a exiting the condensing unit 10 as an alternative to the value provided by temperature sensor with sensor and controller 21 c. Suitable aqueous dew point analyzers may be selected from available instruments including capacitive element, Fourier Transform Infrared spectroscopy (FTIR), mass spectroscopy, or other suitable instruments. The mole fraction of water in the gas phase is determined from the volume concentration reported by sensor 305 divided by the sum of volume concentrations reported by solvent concentration sensor 304 and aqueous dew point sensor 305. The resulting measured mole fraction value is then compared to a set point value in controller 302 to determine an output position value for motorized valve 306 by conventional control loop methods, such as proportional (P) control action, or proportional+integral (PI) control action, or proportional+integral+derivative (PID) control action. Preferred set point values for gas-phase mole fraction of water have been found to be in the range of 0.95 to 0.995, most preferably in the range of 0.98 to 0.99 in the case of NMP solvent.

Alternatively, control of the injection of steam or water spray to increase humidity of stream 70 a may be based on measurement of the composition of the condensed liquid stream in drain line 37 by means of analyzer 37 a. Injection of steam or water in the air entering the condenser increases the fraction of water in the condensed liquid stream while reducing the concentration of solvent in the air exiting the condenser coil. Accordingly, the fraction of water in the condensed liquid correlates to the concentration of solvent exiting the condenser coil as determined by VLE predictions, or preferably by experiment results. The composition of the condensed solvent may be measured continuously or in batch-wise samples for water content along with organic species being recovered. Continuous measurement devices may include analytical instruments such as Fourier Transform Infrared (FTIR), mass spectroscopy, electrical conductivity, or other suitable instruments. In addition to the previous instruments, batch wise measurement of water content may include Karl Fischer titration.

As previously mentioned, condensing effectiveness of organic solvent species such as NMP on condensing coil 20 may be enhanced by co-condensing an amount of water, which advantageously lowers the concentration of solvent leaving condensing region 16 in streams 30 and 72 a. Steam or water is injected through injector 303 and is co-condensed on coil 20. The condensed liquid containing both water and organic solvent is collected in sump basin 36 and discharge in drain line 37. Analyzer 37 a measures the water content of the condensed liquid stream. Desired content of water in the condensed liquid has been found to range from 0.1 to 0.8 mole fraction for enhancement of the condensing effectiveness with the most preferred being 0.2 to 0.5 mole fraction water.

In certain cases it is undesirable to raise the water content of the exit gas 72 a above a certain level as in the case of stream 72 a providing drying air to a lithium ion battery electrode drying operation. In such cases it may be necessary to remove at least a portion of the water content such as by means of a desiccant adsorber in stream 72 b, or more preferably in stream 72 a. For example, the removal of water in a continuous stream is readily accomplished by traversing gas through a desiccant sorbent wheel unit such as offered by Seibu Giken. FIG. 7 shows a preferred embodiment wherein gas flow 72 a enters continuously rotating sorbent wheel 750 removing a portion of the water vapor. A pie-shaped portion of desiccant sorbent wheel 750 adsorbs water from stream 72 a and is subsequently rotated into the desorption zone where heated stream 784 is passed through the sorbent media to desorb the water previously captured from stream 72 a. The desorbed water, now in stream 782, is passed through a condenser device 780 where a cooling media traverses through coil 790 causing the water vapor to condense on said coil. Cooling media may be a brine solution of water or water and glycol from a chiller unit as known in the process cooling industry. In continuous operation, condensed water drains from coil 790 and is collected in the lower housing of condenser 780 and drained through discharge pipe 781. The condensed water is disposed of as wastewater or may be recycled to the process, for example to provide water spray or steam feeding injector 303. Gas stream 783 exiting the condenser 780 is re-heated in heater 785 to provide the preferred temperature for desorption in heated stream 784 as it traverses the desorption section of desiccant wheel 750. Heater 785 may be an electric resistance coil, hot water or steam coil, or other suitable thermal fluid. The pie-shaped sector of now dried sorbent media is rotated into the sorption zone receiving humid flow 72 a removing a portion of its moisture. The aqueous dew point of the exiting stream 730 may optionally be measured by aqueous dew point measurement instrument 310. Preferred aqueous dew point in stream 730 is typically in the range of −60° F. to −20° F. to provided dry air back to the drying process.

The described water injection method may also be used in any of the condensing arrangements of FIGS. 1 through 5 discussed previously. Other condensing plenum arrangements are also possible in place of condensing chamber 10 and may benefit from the moisture injection method described.

Turning now to FIG. 8, pollution control section 40 is comprised of one or more concentrator units 50 and 50′ in series followed by one or more passive (static) bed sorbent units 52 and 52 a in series. Although two exemplary concentrator units and two exemplary static bed units are shown, it can be appreciated that additional concentrator units and passive bed units may be added to pollution control section 40 in order to meet emission requirements in discharge stream 30 f. Stream 30 exits the condenser section 10 and is tempered by introducing heated gas stream line 29 through mixing valve set and control system hardware 31 to obtain the desired set point temperature in stream 30 a before entering the first concentrator unit 50 of pollution control section 40. The first concentrator unit 50 removes a majority of solvent in stream 30 a while a second concentrator unit 50′ removes additional solvent. Additional concentrators may optionally be added to this sequence until the level of solvent remaining in stream 30 d is reduced to a preferred level. The preferred design level of solvent in stream 30 d is determined by identifying the optimal economic operation of the solvent recovery steps in relation to the consumption of sorbent media in the ongoing operation of fixed bed adsorbers 52 and 52 a. In general, the operating expense of concentrator units is predominately the energy costs for fans, heaters and coolant. If the value of recovered solvent is high, the selection favors additional concentrator units. On the other hand, the predominant operating expense of passive adsorber beds is the cost of replacement sorbent, typically activated carbon. For example, capture and recovery of solvent in concentrator units may be favorable down to exit concentrations between 1 ppm and 10 ppm. Once the solvent concentration of the gas stream is reduced to less than 10 ppm, preferably less than 1 ppm, the life of the sorbent charge in a fixed bed carbon adsorber downstream of the last concentrator is often quite long and hence a favorable choice. Further, passive sorbent beds can reliably meet stringent environmental emission requirements by adding additional sorbent vessels in series. Final emission levels of less than 1 ppm may be attained with embodiments of multiple adsorbers place in series.

In an example case, the concentration of solvent in gas stream 30 exiting the condensing region 16 at a temperature of −2° C. is 50 ppm on a volume basis. Stream 30 a is comprised of stream 30 and heated stream line 29 and tempered to a preferred mix temperature in the range of 10 to 20° C., most preferably 15° C. by control of the split-ratio of stream lines 29 and 30. Tempering control system 31 is comprised of a temperature sensor sensing the gas temperature of the mixed stream 30 b, a temperature controller receiving the sensor signal from said temperature sensor, said controller in communication with tempering flow valves in each of the gas stream lines 29 and 30, said controller positioning said tempering valves to result in a mix temperature of stream 30 a equal to the set point value of 15° C. Heated stream line 29 is drawn from the exit of re-heating region 17 at a temperature of 45° C., or alternatively, from the heated stream 72 b exiting economizer heat exchanger 101. The split-ratio of flow of line 30 to flow of line 29 is 2:1 on a normal volume flow basis of 0° C. and 1 atmosphere. Split ratios may fall within the range of 1:2 up to 10:1 during typical dynamic process control operation. Stream 30 a joins return stream 83 from concentrator condenser 80 as stream 30 b to feed concentrator 50 at nominally 15° C. and 50 ppm solvent concentration. The sorbent wheel of concentrator 50 adsorbs solvent and the solvent concentration of stream 30 c is reduced to 5 ppm while the solvent is concentrated in stream 82 and passed through condenser 80 to condense and recover solvent. In this example, in order to prolong the life of the sorbent in the downstream passive bed vessels, it is desirable to further reduce the solvent concentration of stream 30 c by concentrator before final cleaning in passive adsorbent beds. Therefore, stream 30 c is passed through second concentrator 50′ after combination with return stream 83 a from condenser 80 a. The sorbent wheel of concentrator 50′ adsorbs solvent and the solvent concentration of stream 30 d is reduced to 0.5 ppm while the solvent is concentrated in stream 82 a and passed through condenser 80 a to condense and recover solvent. In this fashion, multiple steps of concentration may be carried out until the exit concentration from the last condenser will not saturate the sorbent in the following passive sorbent bed too quickly. In this example, 0.5 ppm is sufficiently low to allow a desired operational life of sorbent in bed 52 of up to one year before replacement of the sorbent charge to avoid solvent breakthrough is reached. Sorbent bed 52 reduces the solvent concentration in stream 30 e to 0.05 ppm. Further reduction of solvent is desired before exhausting stream 30 f to atmosphere. Second sorbent bed 52 a receives flow stream 30 e from bed 52 to further reduce the solvent concentration to less than 0.01 ppm. Operational life before replacement of the sorbent charge in bed 52 a is several years. If lower exit concentrations are required, additional sorbent beds may be placed after beds 52 and 52 a in like fashion.

EXAMPLE 1

With reference to FIG. 7, stream 70 a is composed of air with 2000 ppm by volume of NMP vapor and has an aqueous moisture dew point of −40° C. equating to 190 ppm of water by volume. The target exit concentration of NMP is just below 50 ppm. Condenser temperature 21 c is set to run at 2° C. condensing temperature. With no steam injection, the exit concentration is 66 ppm NMP. Steam is injected by activating control loop 302 and adjusting the set point to obtain an exit concentration from the condenser just less than 50 ppm NMP. To reach just below 50 ppm NMP the dew point setting to controller is adjusted by trial and error to −11° C. dew point resulting in a measured concentration of 47 ppm at analyzer 304. Accordingly, in this process steam is added to the stream 70 a via injector 303 through motorized control valve 306 in communication with controller 302 at a rate necessary to reach an aqueous dew point of −11° C. at sensor 301. The resulting exit concentration of NMP in stream 72 a is 47 ppm by volume and 15 percent of the injected water vapor is condensed with the condensed NMP.

By comparison, in order to achieve less than 50 ppm NMP with no steam injected with stream 70 a containing 2000 ppm by volume of NMP and at an aqueous dew point of −40° F., the required condensing temperature to obtain 47 ppm of NMP in the exit stream 72 a is −2° C. This requires 15% to 20% more cooling wattage in the plant chiller unit.

EXAMPLE 2

Again with reference to FIG. 7, stream 70 a is composed of air with 2000 ppm by volume of NMP vapor and has an aqueous moisture dew point of −40° C. equating to 190 ppm of water by volume. The target exit concentration of NMP is just below 50 ppm. Condenser temperature 21 c is set to run at 2° C. condensing temperature. With no steam injection, the exit concentration is 66 ppm NMP. Steam is injected by activating valve 306 and adjusting the injection rate to obtain an exit concentration from the condenser just less than 50 ppm NMP at analyzer 304. Accordingly, the steam injection rate is manually adjusted until a value of 47 ppm is observed at analyzer 304. In this condition the concentration of water in the condensed liquid in drain pipe 37 measured by analyzer 305 is 0.19 mole fraction water (3.9% by weight). In as much as analyzer 304 is a in some cases a temporary (infrequent) measurement, while analyzer 305 is used frequently for process quality control, the data available from analyzer 305 may now be used as a process set point. Accordingly, steam is added to the stream 70 a via injector 303 through actuated control valve 306 at a rate necessary to reach a mole fraction value of 0.19 as measured at analyzer 305. The resulting exit concentration of NMP in stream 72 a is expected to remain according to VLE physics at approximately 47 ppm volume while the above process parameters are held without the need of measuring exit concentration directly with an additional instrument.

Such conditions as in this example are best determined by experimental testing, especially when targeting very low (e.g. <100 ppm) solvent concentration in the exit air stream. Calculations based only thermodynamic vapor-liquid equilibrium data are often inaccurate due to the condensing and formation of aerosol droplets as previously described. The injection of water by steam or other methods described can have the positive effect of reducing the tendency to form fine aerosol droplets that are not readily collected in the condensing coils or demister of condensing region 16.

While various aspects and embodiments have been disclosed herein, other aspects, embodiments, modifications and alterations will be apparent to those skilled in the art upon reading and understanding the preceding detailed description. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting. It is intended that the present disclosure be construed as including all such aspects, embodiments, modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof. 

What is claimed is:
 1. Circulation air conditioner for a recirculating air dryer generating circulation air laden with at least one condensable fluid, comprising: a. at least one main-condenser having a feed port and an exhaust port and at least one main condensation stage, comprising: ii. a condensation chamber being accessible by or permeable for the circulation air, and iii. a cooling coil at least partially arranged inside said condensation chamber and permeated by a cooling medium, iv. whereby the cooling coil of said main condenser is operated with a main cooling medium temperature of 0° C. or less, b. a circulation air feed line being connected to said feed port of said main-condenser and being connectable to a source of condensable fluid laden air for carrying the circulation air, c. a circulation air exhaust line being connected to said exhaust port of said main-condenser, and d. a side-stream off gas extraction line being fluidly connected to at least said condensation chamber of said main condenser, i. whereby a volume flow of a circulation air streaming in said condensation chamber is split into a high volume re-circulation stream leaving the condenser through the circulation air exhaust line and a low volume off-gas side stream.
 2. The circulation air conditioner of claim 1, further comprising at least a pre-condenser with at least one pre-condensation stage a. being placed in the circulation air stream upstream the main-condenser b. and comprising i. a pre-condenser condensation chamber being accessible by or permeable for the circulation air, and ii. a cooling coil at least partially arranged inside said pre-condenser condensation chamber and permeated by a pre-cooling medium, iii. whereby the pre-cooling medium has a temperature higher than the main cooling medium temperature.
 3. The circulation air conditioner of claim 2, whereby said pre-condenser and said main-condenser are enclosed in a common condenser housing.
 4. The circulation air conditioner according to claim 1, further comprising a pre-cooling heat exchanger being arranged upstream said pre-condensation stage to already reduce a temperature of the streaming in circulation air and/or a reheating heat exchanger being arranged downstream of the main condensation stage.
 5. The circulation air conditioner of claim 4, wherein the pre-cooling heat exchanger and the re-heating heat exchanger are thermally coupled by the exchange of a heat transfer medium such as water, brine or suitable thermal fluid and/or thermally coupled by a thermocouple or heat pipe.
 6. The circulation air conditioner according to claim 1, further comprising an air pollution control unit being fluidly connected to said side-stream off gas extraction line and having at least one adsorptive concentrator with a gas exhaust and a desorption exhaust as a first pollution control stage and at least a second pollution control stage being selected from the group consisting of a filtration device, an absorptive concentrator, a thermal oxidizer, and a catalytic device.
 7. The circulation air conditioner according to claim 6, wherein the second pollution control stage comprises of an adsorptive concentrator being fed by said gas exhaust of the first pollution control stage and having a gas exhaust and a desorption exhaust.
 8. The circulation air conditioner of claim 6 wherein the desorption exhaust of at least one of the adsorptive concentrators is connected to a desorption line, which is connected to a desorbate condenser, whereby a gas exhaust of said desorbate condenser is feed back into the side-stream off gas extraction line.
 9. The circulation air conditioner of claim 7 wherein the desorption exhaust of at least one of the adsorptive concentrators is connected to a desorption line, which is connected to a desorbate condenser, whereby a gas exhaust of said desorbate condenser is feed back into the side-stream off gas extraction line.
 10. The circulation air conditioner of claim 6 wherein the second pollution control stage comprises at least one activated carbon filter.
 11. The circulation air conditioner of claim 1, further comprising an injector in fluid communication with said circulating air feed line.
 12. The circulation air conditioner of claim 11, wherein said injector has a valve, said circulation air conditioner further comprising a dew point sensor for sensing an aqueous dew point temperature value of said condensable fluid laden air entering in said circulating air feed line, a temperature sensor for sensing the temperature of the fluid exiting said at least one main-condenser, and a controller in communication with said dew point sensor and said temperature sensor and operative to control said valve to cause said injector to inject water or steam into said circulating air feed line when said dew point sensor senses an aqueous dew point lower than a predetermined value.
 13. The circulation air conditioner of claim 12, wherein said predetermined value is based on the temperature of the fluid exiting said at least one main-condensation sensed by said temperature sensor.
 14. The circulation air conditioner of claim 11, wherein said injector has a valve, said circulation air conditioner further comprising a solvent concentration analyzer for analyzing the solvent concentration of fluid in said circulation air exhaust line, a dew point sensor for sensing the humidity of the fluid in said circulation air exhaust line, and a controller in communication with said solvent concentration analyzer and said aqueous dew point sensor for calculating a mole fraction of water in said fluid exiting said at least one main-condenser and operative to control said valve to cause said injector to inject water or steam into said circulating air feed line when said mole fraction is lower than a predetermined value.
 15. The circulation air conditioner of claim 1, further comprising a desiccant adsorber in fluid communication with said circulation air exhaust line.
 16. A method for conditioning a circulation air laden with at least one condensable fluid, comprising: a. introducing the circulation air at a first volume flow and intake temperature level well above 0° C. to a main condenser with at least one main condensation chamber; b. gradually cooling the circulation air to a main temperature level of 0° C. or less inside the main condensation chamber; c. splitting the volume flow of the circulation into a high volume re-circulating flow and low volume off-gas side stream after reaching a second temperature level; and d. providing the high-volume re-circulating flow to a circulation air intake of a dryer.
 17. The method according to claim 16, further comprising providing a condenser to cool the circulation air according to step b), which comprises at least one cooling coil containing a cooling medium, and whereby the cooling medium enters the coiling coil at a far end side of a circulation air entry with entry temperature of 0° C. or less and is heated while traveling through the cooling coil in a counter-flow direction to the circulating air.
 18. The method according to claim 16, further comprising: a. feeding the circulation air at the first volume flow to a pre-condenser upstream of said main condenser with a pre-condensing temperature level below a first temperature level and well above said main temperature level ; b. gradually cooling the circulation air to said pre-condensing temperature level in the pre-condenser; and c. providing the cooled circulation air to an intake of said main condenser.
 19. The method according to claim 17, further comprising: a. feeding the circulation air at the first volume flow to a pre-condenser upstream said main-condenser with a pre-condensing temperature level below a first temperature level and well above said main condensing temperature; b. gradually cooling the circulation air to said pre-condensing temperature level in the pre-condenser; and c. providing the cooled circulation air to an intake of said main condenser.
 20. The method according to claim 16, further comprising pre-cooling said circulation air upstream of said main condenser.
 21. The method according to claim 16, further comprising re-heating said circulation air downstream of said main condenser.
 22. The method according to claim 17, further comprising re-heating said circulation air downstream of said main condenser.
 23. The method according to claim 16, further comprising: a. feeding the off-gas side stream to an at least two-stage air pollution control device; b. collecting and increasing the concentration of residual condensable fluid in an adsorptive concentrator as a first pollution control stage; c. subsequently treating a remaining off gas stream in a second air pollution control device as a second stage further downstream of the first stage to a level of concentration of residual condensable in the air well below a predetermined limit.
 24. The method of claim 23, wherein said predetermined limit is 1 mg/Nm³.
 25. The method according to claim 23, wherein said second air pollution control device is a second adsorptive concentrator.
 26. The method according to claim 23, wherein said second air pollution control device is a filtration device.
 27. The method according to claim 26, wherein said filtration device comprises an active carbon filter.
 28. The method according to claim 16, further comprising adding humidity to said circulation air prior to gradually cooling the circulation air.
 29. The method according to claim 28, further comprising sensing the dew point of said circulation air prior to its provision to said main condenser, and sensing the temperature of said high volume re-circulating flow, wherein the amount of humidity added to said circulation air is based on the sensed temperature of said high volume re-circulating flow.
 30. The method of claim 28, further comprising sensing a dew point temperature value of said circulation air prior to introduction of said circulation air into said main condenser; sensing the temperature of fluid exiting said at least one main condenser, and controlling the introduction of humidity in response to the sensed dew point temperature and the sensed temperature of fluid exiting said at least one main condenser.
 31. The method of claim 28, further comprising analyzing a solvent concentration of fluid in circulation air exhausted from said main condenser; sensing humidity of the fluid in said circulation air exhausted from said main condenser; calculating a mole fraction of water in said air exhausted from said main condenser; and introducing humidity into said circulating air entering said main condenser in response to when said mole fraction is lower than a predetermined value.
 32. The method of claim 17, further comprising controlling the temperature of the air exiting the cooling coil to a pre-determined set point with a first controller in control communication with a valve and actuator positioned in a conduit admitting fresh cooling medium to said cooling coil, wherein said pre-determined set point of the air exiting said cooling coil is selected to reach a desired target concentration of humidity of the air exiting said cooling coil.
 33. The method of claim 17, further comprising controlling the temperature of the air exiting the cooling coil to a pre-determined set point with a first controller configured to calculate a temperature set point for the entering cooling medium, said first controller being in communication with a second controller which receives the pre-determined set point from said first controller in a cascade control arrangement; said second controller being in control communication with an actuator and valve to control the measured temperature of the cooling medium entering said cooling coil to said pre-determined set point of said first controller.
 34. The method of claim 17, further comprising measuring the temperature of the cooling medium exiting the coil and inputting said measurement to a controller in communication with a flow control device, and comparing said measurement to a pre-determined set point value, and adjusting the flow control device in response to said comparison, thereby controlling the flow rate of said cooling medium.
 35. The method of claim 17, further comprising measuring the temperature of the cooling medium exiting the coil and inputting said measurement to a third controller in communication with a fourth controller which receives a flow set point from said third controller in a cascade control arrangement; said fourth controller controlling the speed of said flow control device to control the measured flow of the cooling medium entering the coil to the inputted set point of said third PID controller.
 36. The method of claim 16, wherein the coolant media exit temperature is held to a value less than the exit air temperature from the coil.
 37. A method for conditioning a circulation air laden with at least one condensable fluid, comprising: a. introducing water into said circulation air; b. introducing the circulation air at a first volume flow and intake temperature level well above 0° C. to a main condenser with at least one main condensation chamber; c. gradually cooling the circulation air inside the main condensation chamber; d. splitting the volume flow of the circulation into a high volume re-circulating flow and low volume off-gas side stream after reaching a second temperature level; and e. providing the high-volume re-circulating flow to a circulation air intake of a dryer.
 38. The method of claim 37, wherein said water comprises steam.
 39. The method of claim 37, wherein the main temperature level of said condenser is higher than a main temperature level required to condense an equivalent amount of said at least one condensable fluid in the absence of the addition of steam into said circulation air. 